For over two centuries, the blast furnace has been perhaps the principal apparatus used in the iron and steel industry and its development, not only revolutionized the production of iron and steel, but also allowed such production substantially continuously and in large volumes to the point that, although considerable research has been invested in alternative approaches to the reduction of iron ores, the blast furnace generally remains unequalled.
As is well known, in a blast furnace a charge of sintered iron ore, limestone and/or other slag formers, and coke descends from the top (or throat) of the blast furnace, while a bath of hot metal is formed at the bottom thereof, overlain by a slag layer.
Blast delivered by hot stoves is blown by tuyeres somewhat above the slag layer and reacts with the coke of the charge to produce inter alia carbon monoxide which acts as a reducing agent within the furnace and the exothermicity of the various reactions which take place within the furnace provides the necessary heat to melt the metal formed by the reduction of its ore.
In spite of the fact that blast furnaces have been developed with a high degree of sophistication, in at least one respect they still are deficient. It is not possible, utilizing conventional techniques, to alter at short notice their output.
Put otherwise, the blast furnace has, :n the past, been an apparatus which has been capable of modification as to output only over long periods of time and has been incapable of responding to daily or even hourly changes in demand or required output.
If the blast furnace is operated at a fraction of its capacity, for example, its output can only be increased after a considerable lag from the time at which the demand increase is noted. Furthermore, if the blast furnace is operated at less than full capacity, it tends to operate inefficiently since a full height of charge is usually required for effective pre-heating and reduction.
When the blast furnace is operated at its maximum or rated capacity, however, brief intervals of increased demand can not be satisfied and it is either necessary to provide another blast furnace or auxiliary equipment to satisfy the increased production need, at high capital cost.
The variability obtainable by simply controlling the blast is quite limited and generally, the increased demands are so sporadic as not to warrant the capital expenditure for start-up of additional blast furnaces or their construction.